I wouldn't "have the crank fully checked out or rebuilt first" as it'd cost about as much as decent entire 2nd hand engine and quadruple your build cost. It's very unlikely a crank is going to die and destroy something so just leave it if you value your wallet. If/when it does go you can cross that bridge then.

A replate and new piston or a 2nd hand barrel and piston (and a new oil pump seal) is all that is required here to see you back on the road, I've had this sort of drama before (left the jetting too lean after a summer trackday and popped down to the chip shop) and run the bike again 2 hours later. Andy can half that time. Steve can probably do them at the roadside.

Yes, there is a bit of play at the big end (well there is on mine) and if it's seized it's quite likely that some metal has smeared round the pin, more than a little gentle persuasion is not unusual.
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